Which of the following is NOT a form of death penalty used in the United States?

Life imprisonment is not a form of the death penalty used in the United States. The death penalty refers specifically to the government-sanctioned execution of individuals found guilty of certain crimes, typically severe felonies such as murder. The other options—lethal injection, electric chair, and gas chamber—are all methods employed in carrying out capital punishment.

Lethal injection is the most commonly used method today, seen as a more humane approach compared to older methods. The electric chair and gas chamber have also historically been utilized as means for execution. In contrast, life imprisonment represents a sentence where individuals are incarcerated for a long duration, potentially for the rest of their lives, without the possibility of execution. Thus, while life imprisonment is a significant punishment, it fundamentally differs from the concept of the death penalty.